在Java中并行或异步下载多个文件 [英] Downloading Multiple Files Parallelly or Asynchronously in Java

问题描述

在这里，我试图一个接一个地下载多个文件:

Here I am trying to download multiple files one after another:

环境-Java 1.6

Environment - Java 1.6

public List<Attachment> download(List<Attachment> attachments)
{
  for(Attachment attachment : attachments) {
    attachment.setDownStatus("Failed");
    String destLocation = "C:\Users\attachments";
    try {
        String attUrl = attachment.getUrl();
        String fileName = attachment.getFileName();            
        URL url = new URL(attUrl);
        File fileLocation = new File(destLoc, fileName);
        FileUtils.copyURLToFile(url, fileLocation);
        if(fileLocation.exists()) {
           attachment.setDownStatus("Completed");
         }
       } catch(Exception e) {
          attachment.setDownStatus("Failed");
       } finally {
          attachment.setDestLocation(destLocation);
       }
   }
  return attachments;
}

我正在从提供的URL( http ://cdn.octafinance.com/wp-content/uploads/2015/07/google-hummingbird.jpg ).

I am downloading the file from provided URL (http://cdn.octafinance.com/wp-content/uploads/2015/07/google-hummingbird.jpg).

FileUtils.copyURLToFile(url, fileLocation);

上面的代码完美地完成了下载工作，没有任何问题.

The above code does its downloading job perfectly, without any issues.

我的问题:
如果附件列表更多，则将花费更多时间，因此我想使其成为异步或并行过程，而不是顺序下载.

My Problem:
If the list of attachments are more it will take more time, so I would like to make it an asynchronous or parallel process instead of downloading sequentially.

推荐答案

实际上，经过仔细查看，鲍里斯(Boris)的代码是有错误的，确实有时不会设置任何内容.这是一个更好的版本，可以解决此问题:

Actually, after carefully looking, Boris' code is faulty and will indeed not set some stuff sometimes. Here's a better version that fixes that:

public List<Attachment> download(List<Attachment> attachments) {
  ExecutorService executorService = Executors.newCachedThreadPool();
  List<Future<Attachment>> futures = new ArrayList<Future<Attachment>>();
  for (final Attachment attachment : attachments) {
    futures.add(executorService.submit(new Callable<Attachment>() {
      @Override
      public Attachment call() throws Exception {
        return doDownload(attachment);
      }
    }));
  }
  for (Future<Attachment> future: futures) {
    try {
      future.get();
    } catch (Exception ex) {
      // Do something
    }
  }
  return attachments;
}

private Attachment doDownload(Attachment attachment) throws Exception {
  attachment.setDownStatus("Failed");
  attachment.setDestLocation("C:\\Users\\attachments");
  String attUrl = attachment.getUrl();
  String fileName = attachment.getFileName();
  URL url = new URL(attUrl);
  File fileLocation = new File(attachment.getDestLocation(), fileName);
  FileUtils.copyURLToFile(url, fileLocation);
  if (fileLocation.exists()) {
    attachment.setDownStatus("Completed");
  }
  return attachment;
}

但是，鉴于您的Attachment结构及其使用方式，这绝对不是最佳选择.我没有解决这个问题:我只是回答了这个问题.

However, this is absolutely not optimal given your structure of Attachment and how you use it. I did not fix that: I only answered the question as it was asked.

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